This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-33392, filed on Oct. 31, 2000, the entire contents of which are incorporated herein by reference.
(1) Field of the Invention
The present invention relates to a color image forming method and a color image forming apparatus and more particularly to a color image forming method and a color image forming apparatus for superimposing a plurality of images on an image forming unit and then transferring the color images onto a transfer medium in a batch.
(2) Description of the Related Art
In an image forming apparatus for forming a color image on an image transfer medium such as a recording form, there are various systems available. For example, one of them is a tandem system for arranging image forming units for forming a single-color toner image in correspondence to three colors of yellow, magenta, and cyan or four colors including black whenever necessary in the direction of recording-form transfer and sequentially superimposing a single-color image on a recording form so as to form a color image. The other one is a multiple-development system for installing an exposure means and developing units of three colors or four colors including black on one image forming unit and superimposing each single-color image on the image forming unit so as to form a color image.
Further, there are two multiple-development systems for superimposing and developing a plurality of images available as shown below. One of them is a system for installing one exposure means on an image forming unit, forming a latent image once every rotation of the image forming unit, developing one color, and superimposing each single-color image on the image forming unit by three rotations or four rotations including black so as to form a color image. The other one is a system for installing exposure means and developing units for three colors or four colors including black on an image forming unit, forming and developing a latent image of each color during one rotation, and superimposing each single-color image so as to form a color image.
In the aforementioned tandem system, the respective image forming units are generally arranged continuously in the direction of recording form transfer at an interval of a predetermined distance, so that a plurality of images cannot be printed at the same time and when an output image formed by a preceding image forming unit is transferred by the distance between the image forming units in the direction of recording form transfer from the preceding image forming unit, it is superimposed from above by the next image forming unit so as to form an image.
Therefore, depending on assembly precision and mounting position errors of each image forming unit, the respective single-color images may not be precisely formed and superimposed. Further, images may be shifted due to an uneven recording form transfer speed. Due to these causes, as one of the disadvantages of the tandem system, difficulty in keeping the image superimposition precision high may be cited.
In order to correct such a relative displacement of each single-color image, conventionally, for example, as described in Japanese Patent Application Laid-Open 8-278680, predetermined detection marks are formed on the recording form transfer belt so as to be shifted by a predetermined distance in the respective image forming units, and the variation of each detection mark from the predetermined distance is measured by a detector such as an optical fiber sensor or a line sensor, and the displacement is corrected by adjusting the image drawing timing of each image forming unit or controlling the recording form transfer speed.
On the other hand, in a multiple-development system having one exposure means, a latent image of the first color is formed on the image forming unit by the image exposure means and the latent image of the first color is developed by the development means installed on the downstream side thereof. When the image forming unit makes one rotation and reaches the position of the image exposure means again, a latent image of the second color is formed on the image forming unit and the latent image of the second color is developed in the same way. In the same way, images of three colors or four colors including black are developed every rotation. Thereafter, the color images are transferred onto the intermediate transfer medium or image transfer medium in a batch.
Since an image is formed on the image forming unit color by color ever rotation by one image exposure means like this, if a latent image is drawn on the same position on the image forming unit every rotation, no displacement of each color is generated. Therefore, to reduce the displacement of each color in this system, for example, as described in Japanese Patent Application Laid-Open 6-1002, a position detection mark is provided on an image forming unit and on the basis of a signal detecting the mark, each image is positioned by drawing each color every rotation. However, the image forming unit must make three rotations or four rotations including black so as to form one color image, so that the system is not suited to increase the color image forming speed.
On the other hand, in a multiple-development system having an image exposure means installed in each development means, a latent image is formed on the image forming unit by the image exposure means of the first color and developed by the development means of the first color installed just on the downstream side of the image exposure means. Thereafter, the image forming unit moves and after the time interval decided by the moving speed thereof and the interval between the first color and the second color, a latent image is formed by the image exposure means of the second color. In the same way, images of three colors or four colors including black are superimposed, thus a color image is formed and transferred onto the intermediate image medium or image transfer medium. In this system, a color image can be formed by one rotation, so that the image forming speed can be increased. Further, the image exposure means and development means can be arranged around the image forming unit, so that there is an advantage that the whole apparatus can be miniaturized.
However, in this system, in the same way as with the tandem system, a displacement of each color is caused by the assembly precision of each image exposure means, the mounting precision between the respective image exposure means, the thermal expansion, and errors with time and a deterioration of the image quality is caused. To reduce the displacement of each color in this system, conventionally, as described in Japanese Patent Application Laid-Open 8-240949, a method is used that a support member is provided around the image forming unit, thus the precision of relative position of each image exposure means is improved.
Further, the inventors proposed a method for developing a mark for detecting a superimposition displacement of images as disclosed in Japanese Patent Application Laid-Open 2000-137358 by a developer of a specific color, thereby improving the detection precision.
However, in the recent request for high speed and high resolution of a color image forming apparatus, a request for high precision for a displacement of each color is increased more and sufficient image superimposition precision cannot be obtained by the conventional proposed system aforementioned.
Furthermore, a detection mark formed on the image forming unit is unnecessary for an image output from the image forming apparatus and it is an image to be removed in the apparatus. Therefore, in Japanese Patent Application Laid-Open 2000-137358, the image transfer medium is separated from the image forming unit and the detection mark is not transferred onto the image transfer medium and removed by the image forming unit cleaner installed behind the transfer position in the rotational direction. The detection mark which becomes unnecessary after detection of the image superimposition displacement can be erased in this way.
The image transfer medium is pressed against the image forming unit at a high press contact load. To prevent an image from disorder due to a relative speed difference between the image forming unit and the image transfer medium at the transfer position, the image transfer medium is driven by the tangential force from the image forming unit. Therefore, the load applied to the image forming unit is mostly a frictional load caused by a loss of the bearing of the image transfer medium.
In a state that an output image is formed actually, the image forming unit is in contact with the image transfer medium, and images formed on the image forming unit are continuously transferred onto the image transfer medium and additionally transferred to a medium such as a recording form, and an image is output.
On the other hand, when an image is formed in a state that the image forming unit is separated from the image transfer medium beforehand not to transfer the detection mark, the image transfer medium is pressed against the image forming unit at a high load, thus the load is greatly reduced compared with that at the time of real image output.
As a result, the rotational speed of the image forming unit is different between a case of forming a detection mark and a case of forming a real image and a problem arises that the superimposition correction is a correction which is very unreliable and meaningless.
As mentioned above, conventionally, to obtain sufficient superimposition precision, a method for forming a mark for detecting a superimposition displacement and adjusting the image position from the variation of the detection mark is used. However, when the image forming unit and the image transfer medium are separated from each other beforehand so as to erase the detection mark which becomes an unnecessary image after ending of variation detection, the rotational speed of the image forming unit is different between a case of forming a detection mark and a case of forming a real image and a problem arises that the displacement correction between an image of the first color and an image of the second color does not function.
The present invention was developed with the foregoing problems in view and is intended to provide a color image forming method for precisely positioning an image of the first color and an image of the second color and suppressing an image displacement.
In an embodiment of a color image forming apparatus of the present invention, the color image forming apparatus has an image forming unit that the surface thereof rotates, a first image forming device arranged around the image forming unit for forming a first detection mark and a first image, a second image forming device for forming a second detection mark and a second image over the first image on the surface of the image forming unit, a transfer mechanism for transferring the first image and second image onto the image forming unit, and a controller for controlling these units, and a detector for detecting the first detection mark formed by the first image forming device and the second detection mark formed by the second image forming device is installed, and the controller corrects a forming position of the first image by the first image forming device or a forming position of the second image by the second image forming device according to a relative position of the first and second detection marks detected by the detector, and the color image forming apparatus has transfer mechanism separation means for keeping the transfer mechanism separated from the image forming unit at least during passing of the first and second detection marks through the transfer mechanism.
The detector may be installed between the transfer mechanism and the second image forming device.
A color image forming method of an embodiment of the present invention is a color image forming method comprising a first image forming step of forming a first image on the surface of a rotating image forming unit by a first image forming device, a second image forming step of forming a second image on the surface of the rotating image forming unit on which the first image is formed by a second image forming device, and a step of transferring the first image and second image formed on the surface of the image forming unit to a transfer mechanism arranged in contact with the image forming unit in a batch, and the color image forming method further comprises a detection step of forming a first detection mark on the surface of the image forming unit by the first image forming device in a state that the transfer mechanism is in contact with the image forming unit, forming a second detection mark on the image forming unit on which the first detection mark is formed by the second image forming device, and detecting the relative position of the first detection mark and second detection mark by a detector arranged at the detection position and a separation step of keeping the transfer mechanism in non-contact with the image forming unit when the first detection mark and second detection mark pass the transfer position, and the first image forming step is performed in a state that the transfer mechanism is in contact with the image forming unit, and the second image forming step is performed by controlling the timing according to the detection result obtained at the detection step in a state that the transfer mechanism is in contact with the image forming unit.
A color image forming method of an embodiment of the present invention is a color image forming method comprising a first image forming step of forming a first image on the surface of a rotating image forming unit by a first image forming device, a second image forming step of forming a second image on the surface of the rotating image forming unit on which the first image is formed by a second image forming device, a first transfer step of transferring the first image and second image formed on the surface of the image forming unit to a first transfer medium arranged in contact with the image forming unit at a first transfer position in a batch, and a second transfer step of feeding a second transfer medium between the first transfer medium and a pressure body arranged in contact with the first transfer medium at a second contact position and transferring the first image and second image transferred onto the intermediate transfer medium in a batch onto the second transfer medium in a batch, and the color image forming method further comprises a detection step of forming a first detection mark on the surface of the image forming unit by the first image forming device in a state that the second transfer medium is in contact with the pressure body, forming a second detection mark on the image forming unit on which the first detection mark is formed by the second image forming device, transferring the first detection mark and second detection mark onto the first transfer medium in a batch, and detecting the relative position of the first detection mark and second detection mark on the first intermediate transfer medium by a detector arranged at the detection position, a separation step of keeping the second transfer medium in non-contact with the first transfer medium when the first detection mark and second detection mark pass the second transfer position, the first image forming step to be performed in a state that the second transfer medium is in contact with the first transfer medium, and the second image forming step to be performed by controlling the timing according to the detection result obtained at the detection step in a state that the second transfer medium is in contact with the first transfer medium.
A color image forming method of an embodiment of the present invention is a color image forming method comprising a first image forming step of charging the surface of a rotating photosensitive drum by a first charger, selectively exposing the charged surface of the photosensitive drum by a first exposure unit, thereby forming an electrostatic latent image of a first image on the surface of the image forming unit, developing the electrostatic latent image of the first image by a first developing unit for feeding a developer to the electrostatic latent image of the first image, and forming the first image, a second image forming step of charging the surface of the rotating photosensitive drum on which the first image is formed by a second charger, selectively exposing the charged surface of the image forming unit by a second exposure unit, thereby forming an electrostatic latent image of a second image on the surface of the image forming unit, developing the electrostatic latent image of the second image by a second developing unit for feeding a developer to the electrostatic latent image of the second image, and forming the second image, and a step of transferring the first image and second image formed on the surface of the photosensitive drum to an intermediate transfer roller arranged in contact with the photosensitive drum at the transfer position in a batch, and the color image forming method further comprises a first detection mark forming step of charging the surface of the photosensitive drum, selectively exposing the charged surface of the photosensitive drum by the first exposure unit in a state that the intermediate transfer roller is in contact with the photosensitive drum, thereby forming an electrostatic latent image of a first detection mark, developing the electrostatic latent image of the first detection mark by the first developing unit, and forming a visible image of the first detection mark, a second detection mark forming step of selectively exposing the surface of the photosensitive drum on which the first detection mark is formed by the second exposure unit after a predetermined time from forming the electrostatic latent image of the first detection mark, thereby forming an electrostatic latent image of a second detection mark, developing the electrostatic latent image of the second detection mark by the second developing unit, and forming a visible image of the second detection mark, a detection step of detecting a time difference between the first detection mark and the second detection mark passing a detection position by a detector arranged at the detection position, a separation step of keeping the intermediate transfer roller and the photosensitive drum in non-contact with each other when the first detection mark and second detection mark pass the transfer position, the first image forming step to be processed in a state that the intermediate transfer roller is in contact with the photosensitive drum after the detection step, and the first image forming step of calculating the time until the exposure area exposed by the first exposure unit is exposed by the second exposure unit from the predetermined time and detected time difference and being performed after the calculated time from start of the first image forming in a state that the intermediate transfer roller is in contact with the image forming unit.